Absorption heat pump with burner power modulation

ABSTRACT

An absorption heat pump in which to improve heat efficiency when under desorber power modulation conditions, heat is exchanged between the rich solution, before it enters the desorber, and the poor solution withdrawn from the desorber, before this poor solution is fed into the desorber.

The present invention relates to an absorption heat pump with burner power modulation.

In known heat pumps used for heating, the facility to modulate the thermal power delivered (and that supplied to the generator) makes the use of such machines much more flexible. This results in a greater ease of coupling to the system thermal load, delivered temperature precision and better efficiency related to the drastic reduction of transients.

Current market research suggests that efficiency requirements for modulation heat pumps have risen to COP values greater than 1.6, with seasonal performance greater than 1.3. Burner modulation can hence reach 20% of the rated power.

A heating heat pump with modulation is already available in absorption technology. In this case the machine power can be modulated to 50% of the burner power.

Fixed throughput throttling members are used to modulate the generator power of existing absorption heat pumps, hence accepting sub-optimal operating conditions. In this respect, the high flow of poor solution leaving the generator when supplied with a reduced heat quantity results in an increase in the contained refrigerant concentration of up to values from 10 to 20 points of mass concentration. Such a high solution concentration results in flash evaporation (effect related to refrigerant boiling consequent on a sudden pressure drop to a value less than the saturation value of the liquid at the given concentration) downstream of the throttling member, with a downstream liquid temperature reduction even of 35° C. The result of this is lesser heat recovery in the heat exchanger and hence a lower cycle efficiency.

In other modulating pumps, for example those described in U.S. Pat. No. 6,748,752, when the thermal generator power is modulated, the poor solution flow is controlled by modulating valves (variable flow restrictors) to maintain the cycle operating parameters under optimum conditions. These devices present, inter alia, the following drawbacks.

It is difficult to construct modulating valves because of the precision required in regulating fluid flows generally of very low absolute value (from 45 kg/h at full load operation to 9 kg/hr with partial load operation).

They are also sensitive to dirt originating from the bottom of the vapour generator.

They also have a high cost and require a further actuator, typically electronic/electromechanical.

A further drawback deriving from modulation is that the low liquid flow (when the valve allows only low flow passage) means that neither film heat exchangers nor traditional heat exchangers are able to operate correctly.

In this case, efficiency falls to very low values (so nullifying flow modulation)

An object of the present invention is to provide a heat pump in which the efficiency does not fall greatly when the generator power is modulated, down to 20% of the rated power.

This and other objects are attained by a heat pump formed in accordance with the technical teachings of the accompanying claims.

Advantageously the pump in question is also able to operate with excellent efficiency even under extreme conditions, i.e. at very low external temperatures and if very hot water (>65° C.) is required. Further characteristics and advantages of the invention will be apparent from the description of a preferred but non-exclusive embodiment of the heat pump, illustrated by way of non-limiting example in the accompanying drawings, in which the single figure shows a simplified scheme of the heat pump of the present invention.

With reference to said figure, this shows a heat pump indicated overall by the reference numeral 1.

It operates with a cycle using a first fluid, in this specific case ammonia, as refrigerant, this being absorbed in a second fluid which in this case is water. The absorption heat pump comprises a conventional generator 2 or desorber presenting a finned gas burner 35, which feeds a conventional plate column 36. The burner 35 comprises a control module 35A for the delivered power.

The plate column 36 is connected to a rectifier 33, described hereinafter. The rectified vapour outlet of the generator is connected via a first line 3 to a condenser 4 of conventional type, positioned in heat exchange contact with a transmission fluid which feeds the heating plant. This fluid is typically water fed into the plant by a pump, not shown.

A countercurrent heat exchanger 34 is provided downstream of the condenser 4 in a second line 6 connecting the condenser to an evaporator 34 via a lamination valve 5, to exchange heat with the vapour circulating through a third line 8 connecting the evaporator 7 to an inlet 10B of an absorber 10. A further lamination valve 36 is provided upstream of the heat exchanger 34.

As already stated, an evaporator outlet 7B is connected by the third line 8 to an inlet 10B for vapour from said first fluid into the absorber 10, and specifically into a mixing zone 9.

The absorber 10 comprises a rich solution outlet 10C (ammonia absorbed in water) connected to a heat exchanger 13 in heat exchange contact with the transmission fluid of the heating plant.

An outlet 13B of the heat exchanger is connected to the suction side of a conventional pump 14, the delivery side of which is connected via a fourth line to an inlet 16 of a circuit 16A, 16B in heat exchange contact with the absorber 10.

The fourth line 15 lies in heat transmission contact with the rectifier 33 from which the rich ammonia solution subtracts heat to facilitate condensation of water vapour.

The circuit 16A, 16B subtracts heat from the absorber to hence transfer it to the rich solution originating from the pump 14 before being fed into the generator 2. This circuit is divided into two parts only for reasons of description. In this respect, in the first part of the circuit 16A the rich solution rises its temperature, while in the second part 16B the ammonia present in the solution begins to evaporate (at the pressure present in the circuit 16A, B) to essentially anticipate the work done by the generator 2. That part of the absorber involved with the circuit part 16B is commonly known as a GAX cycle.

A fifth line 18 extending from the heat exchanger 10 connects an outlet of the circuit 16A, 16B to an ammonia enriched solution (plus ammonia vapour) inlet 2B of the generator 2.

At the generator base, in proximity to the burner 35, an outlet 2C is provided from which a poor ammonia solution is directed, via a sixth line 19 provided with at least one lamination valve 30, to a poor solution inlet 10A provided in the absorber 10, after yielding heat to the fluids present in the generator in a central portion 2D thereof.

According to the present invention the line 19, which extends from the central portion 2D of the desorber, comprises a lamination valve 41, downstream of which there is an intermediate heat exchanger 40, preferably of the countercurrent type, enabling heat to be exchanged between the poor solution present in the line 19 and the rich solution flowing through the line 18 from the absorber 10 and directed to the desorber 2.

The pressure to which the poor solution is brought by the valve 41 is an intermediate value between the pressure of the generator 2 and the pressure of the absorber 10, enabling flash evaporation at the first valve 41 to be reduced under most working conditions of the heat pump, as the intermediate pressure downstream of the valve 41 means that the solution is below the boiling point. Advantageously, in the intermediate heat exchanger 40 the poor solution yields heat to the rich solution entering the generator. Consequently the poor solution becomes cold. This enables flash evaporation to also be avoided downstream of the valve 30. Downstream of the valve 30 the solution temperature is similar to that which would exist in the case of flash evaporation without the heat exchanger 40, however the solution is all in the liquid phase and, having yielded some heat to the rich solution directed to the absorber, efficiency is improved.

The proposed system provides very high cycle performance when this is under standard conditions (GAX effect), whereas when under modulation conditions it enables a poor solution flow to be maintained such as to always maintain the flow properly distributed and the surfaces of the absorber 10 wetted, the efficiency of this latter being much influenced by the transiting mass flow.

It has been found that a system of this type enables better performance to be obtained than traditional heat pumps which do not present the system herein described.

By way of example, it has been found that for 50% generator modulation an efficiency improvement of about 10% is obtained compared with traditional absorption pumps, this value gradually increasing to 35% for modulation percentages of about 30% of the rated load.

If the intermediate heat exchanger 40 is of the tube-in-tube type, in which the poor solution of the line 19 flows through the annular conduit, the annular passage cross-section can be dimensioned such that when this section is fed with maximum liquid flow, only a small pressure drop (from 0.2 to 0.5 bar, preferably 0.35 bar) occurs at the heat exchanger outlet.

This pressure drop does not negatively influence the full load operation given that the fluid has in any event to pass through the lamination valve 30 which reduces the pressure by a number of bars.

In the limit case of flash evaporation across the first lamination valve 41 (for example for generator loads <25% of rated power) the solution mass flow would remain approximately unvaried, but certainly not the volumetric flow. The volumetric flow (two-phase) in the annular conduit would considerably increase (at least until the flash vapour has recondensed by the cooling due to heat exchange in the heat exchanger 40), to cause a pressure drop of even several bars (depending on the heat exchanger geometry and the extent of flash evaporation).

The second lamination valve 30 has a reduced pressure at its inlet, and as the lamination valves are calibrated orifice plates, the flow through which depends mainly on the pressure drop between their two ends (in addition to their geometry), the flow will be less. Hence an automatic static flow regulator system is obtained.

This system enables extreme modulation conditions and low evaporation pressures to be handled. Hence high performance can be maintained even under extensive modulation, and finally high seasonal COPs.

In contrast to the known art, moving members or electrical controls are not present.

This reduces system complexity, decreases costs and above all increases flexibility.

According to a particular embodiment of the present invention, a system (not necessarily provided and hence optional) is present for maintaining the top of the desorber plate column “colder” and reducing the rectifier load. To achieve this, the flow and/or NH₃ concentration of the rich solution entering the generator 2 is increased. This can be done by bleeding off part of the liquid refrigerant leaving the condenser and mixing it with the rich solution line entering the generator, by using the entrainment effect of a liquid-liquid injector.

According to the optional system, a point 22 for the introduction or feed of condensed vapour (liquid ammonia) into the rich ammonia solution is provided between the inlet 16 of the circuit composed of the first and second part 16A, 16B and the rich solution inlet 2B of the generator.

The introduction point 22 can be arranged in various plant positions, a particularly advantageous one of which is represented in the accompanying figure. A first introduction point is represented by a dashed line and indicated by the reference numeral 22. With this solution the withdrawal line 20 which extends from the withdrawal point 24 extends along the portion 20 and advantageously feeds into the venturi 22 shown in the figure. This is positioned in a circuit portion downstream of the second part 16B of the circuit.

Introducing bled refrigerant into the solution flow “costs” in terms of machine power (refrigerant flow to the evaporator). This cost can be minimized to obtain an advantage under certain conditions. This introduction point is particularly advantageous when located in a point of the circuit 16A, 16B in which the solution present therein has a temperature close to that of the temperature resulting from mixing the two flows, i.e. the refrigerant flow and the solution flow. In this respect, adiabatic mixing of two liquid flows [for example 44% NH₃ in the solution, 99% NH₃ in the refrigerant] results in a flow at a temperature greater than the two inlet temperatures.

This optimum temperature is between 60° C. and 90° C., preferably between 70° C. and 80° C.

If the refrigerant bypass flow is for example 10% of the refrigerant, then ammonia concentration in the rich solution can increase by between 2 and 4%. This implies that the GAX regenerator (second portion 16B of the circuit) begins to reboil the solution at a temperature less by 4° C. and 6° C., compared with when the ammonia concentration in the solution is less.

For example, for an ammonia concentration of 44% in the solution, the boiling temperature at 20 bar is 103° C. By increasing the concentration to 47% with the bypass line 20, 20A, 20B, the boiling temperature falls to 97° C. at the same pressure. The vapour regenerated hence “recovers” the expense of the bypass.

This results in a lowering of the desorber column and rectifier temperature by about 10-15° C., with considerable benefits.

The result is that for equal evaporator power there is a greater “load” at the condenser (which therefore has to be slightly over-dimensioned). However there is a lesser load at the rectifier and generator, which work at lower temperature.

This situation becomes very interesting precisely when high (>65° C.) water temperatures are required from the heating plant, or for generating domestic hot water. In this case, conventional heat pumps generate pressures and temperatures which cause the desorber column to “work” at its limit, so bringing the rectifier load to critical levels, and drastically reducing the refrigerant flow fed to the condenser (also because the GAX regenerator at these high pressures does not regenerate refrigerant vapour). Increasing the heat exchanger surfaces does not improve the situation, while at high temperatures the risk of surface corrosion increases. As previously explained, bypassing the refrigerant increases the rich solution concentrations, so extending system working conditions.

As an alternative to that already described, under determined temperature and pressure conditions the refrigerant can be injected or fed into the rich solution at a point between the first part 16A and second part 16B of the circuit. This solution is not represented in the figure, but the benefits obtainable are substantially the same.

In the two described embodiments, refrigerant injection or feed takes place preferably by means of a venturi, which enables the refrigerant to be “drawn” into the solution. However, injection can be effected by any other suitable means.

In addition to comprising a refrigerant non-return valve 32 for both the above embodiments, the refrigerant feed line 20 can also comprise a solenoid valve or the like which completely excluders the bypass line, hence enabling the heat pump to be used in a completely conventional manner.

It has been seen that by introducing the aforedescribed circuit modification, the heat pump operates under a wide variety of conditions, with much higher efficiencies than conventional heat pumps, especially when these conditions are extreme.

Various embodiments of the invention have been described, but others can be conceived by utilizing the same inventive concept. All the described components can be replaced by technically equivalent elements. Moreover the refrigerant and the liquid in which it is absorbed can be chosen at will in conformity with the necessary technical requirements. 

1. An absorption heat pump comprising; a generator or desorber associated with a device for modulating its power, the desorber for generating, from a first fluid, vapor fed via a first line to a first condenser in heat exchange contact with a transmission fluid, downstream of the condenser there being provided a second line entering an evaporator, the second line 6 comprising at least a first lamination valve, an evaporator outlet being connected by a third line to an inlet for vapor from said first fluid into an absorber, comprising an absorber outlet for an enriched solution of said first fluid absorbed in a second fluid, the absorber outlet being connected to a heat exchanger in heat transmission contact with the transmission fluid, heat exchanger outlet of the heat exchanger being connected to a suction side of a pump, the delivery side of the pump is connected by a fourth line to an inlet of a circuit in heat transmission contact with the absorber, a fifth line connecting said circuit to a rich solution inlet of the generator, the generator having a poor solution outlet connected by a sixth line provided with a lamination valve to a poor solution inlet provided in the absorber, the pump further comprising an intermediate heat exchanger arranged to bring the fluids present in the sixth line and in the fifth line into heat exchange contact, wherein a lamination valve is provided in the sixth line prior to the inlet to the intermediate heat exchanger to lower the poor solution pressure in the intermediate heat exchanger.
 2. A pump as claimed in claim 1, wherein the heat exchanger is of concentric tube type defining an annular passage for flowing the poor solution through the annular passage, the annular passage cross-section sized to provide only a slight pressure drop at the heat exchanger outlet when this annular passage cross-section is traversed exclusively by liquid at the maximum throughput allowed by the heat pump.
 3. A pump as claimed in claim 1, wherein the slight pressure drop is between 0.05 and 0.5 bar.
 4. A heat pump as claimed in claim 1, wherein an introduction point for condensed vapor from said first fluid circulating through the circuit is provided between the inlet of the circuit and the rich solution inlet of the intermediate heat exchanger.
 5. A heat pump as claimed in claim 4, wherein the condensed vapor is withdrawn at a withdrawal point positioned directly downstream of the condenser by a withdrawal line.
 6. A heat pump as claimed in claim 6, wherein a non-return valve is provided in the withdrawal line, between the withdrawal point and the introduction point.
 7. A heat pump as claimed in claim 1, wherein said introduction point is provided between a first portion and a second portion of said circuit.
 8. A heat pump as claimed in claim 7, wherein said introduction point is provided downstream of the second circuit portion outside the absorber.
 9. A heat pump as claimed in claim 5, wherein said withdrawal line comprises a valve arranged to close the withdrawal line.
 10. A heat pump as claimed in claim 1, wherein a rectifier in heat exchange contact with the fluid leaving the pump is between the generator and condenser.
 11. A heat pump as claimed in claim 1, wherein the sixth line is in heat exchange contact with a central portion of the generator.
 12. A method for improving the efficiency of an absorption heat pump according to claim 1 when under desorber power modulation conditions, comprising the step of: exchanging heat between the rich solution, before the rich solution enters the desorber, and the poor solution withdrawn from the desorber, before this poor solution is fed into the desorber, before undergoing the heat exchanging, the poor solution pressure is lowered to an intermediate value between the desorber pressure and the absorber pressure.
 13. A method as claimed in claim 12, wherein refrigerant is bled off downstream of the condenser and mixed with the rich solution after this rich solution has been at least partially heated by the absorber and before the rich solution undergoes heat exchange with the poor solution.
 14. A method as claimed in claim 12, wherein the refrigerant is bled off between the condenser and the evaporator.
 15. A method as claimed in claim 12, wherein the refrigerant is mixed with the rich solution at a point in which the difference between the temperature of the rich solution before the mixing and the temperature resulting from the mixing of the rich solution with the refrigerant is between −5° C. and 5° C.
 16. A method as claimed in claim 12, wherein refrigerant is optionally bled off, wherein said bleeding can be excluded, depending on the pump working conditions. 